基于有限差分法的三峡库首区蓄水前后应力场变化与地震活动研究

Stress field changes and seismic activities before and after the impoundment in the head area of the Three Gorges Reservoir based on the finite-difference method

摘要:

水库蓄水前后构造应力场的变化在水库诱发地震研究中具有重要意义。为了探究三峡水库蓄水后应力场变化及蓄水与水库地震的相关性,以三峡水库库首区九畹溪断裂、仙女山断裂和高桥断裂为研究对象,选取自2003年开始蓄水之后72个月的水位变化数据,模拟研究不同蓄水期库水荷载对三峡库首区有效应力及剪应变增量的影响。基于RHINO软件实现真三维地层建模,并应用有限差分软件FLAC3D模拟不同蓄水期库首区有效应力及地下500 m处剪应变增量的变化。从模拟结果可知,库首区的最大主应力和最小主应力都随着水位的上升而增大;与蓄水前相比,蓄水深度为135、145和177 m时,最大主应力增幅分别为5.8%、13.1%和16.8%,最小主应力增幅分别为20.5%、20.5%和32.6%;主应力数值与蓄水位存在正相关性,蓄水位的增大对地震的发生起到了催化和推动作用,提高了地震风险性。研究结果可为三峡水库诱发型地震预测提供参考。

Abstract:

The changes in the structural stress field before and after reservoir impoundment are of considerable importance in the study of reservoir-induced seismicity. This study focuses on the Jiuwanxi, Xiannvshan, and Gaoqiao faults in the head area of the Three Gorges Reservoir to investigate the stress field changes and the correlation between reservoir impoundment and earthquakes in the Three Gorges Reservoir. The water level change over 72 months since the impoundment in 2003 is selected to simulate the effect of reservoir water loads during different impoundment periods on the effective stress and shear strain increment in the study area. The true 3D geological modeling is implemented using the software RHINO, and the changes in the effective stress of the study area and the shear strain increment at a depth of 500 m underground are simulated using the finite-difference software FLAC3D. Simulation results show that the maximum and minimum principal stresses in the head area increase with the rise in water levels. Compared with those before impoundment, the maximum principal stress is increased by 5.8%, 13.1%, and 16.8% when the impoundment depth is 135, 145, and 177 m, respectively, and the minimum principal stress is increased by 20.5%, 20.5%, and 32.6%, respectively. A positive correlation exists between the principal stress value and the water level, and the change in water level plays a catalytic role in the occurrence of earthquakes. This study can provide a reference for earthquake predictions in the Three Gorges Reservoir.